PTOs are used on many types of vehicles, including on agricultural vehicles such as tractors, to provide power for equipment or implements, such as, for agricultural purposes, combines, mowers, balers, forage harvesters and spreaders.
Modern tractors commonly have horsepower ratings in excess of 100 horsepower. However, the shaft sizes for PTOs have not changed due to the need to maintain compatibility with older equipment and maintain the standardization for PTOs. Thus, the torque output of PTOs for many modern tractors is no longer limited by the tractor horsepower. Rather, the torque output is limited by the strength of the PTO shaft and the failure thereof. In addition to causing PTO shaft failures, the torque produced by the high horsepower tractors can accelerate equipment attached to the respective PTO at a rate which can damage the equipment.
In view of the problems associated with the control of PTO shafts in high horsepower tractors, it was found desirable to provide a PTO clutch control system for protecting PTO shafts from catastrophic failure and for providing PTO shaft accelerations at rates which protect the shafts and attached equipment during clutch engagement.
Typical of such a system is the system of U.S. Pat. No. 5,494,142, which discloses a PTO control system for vehicles, such as farm tractors including a power take-off (PTO) shaft, for supplying rotational motion to an implement of the type which may be stationary or towed by the tractor. Power is transferred to the PTO shaft by a clutch including an input shaft coupled to a power source and an output shaft coupled to the PTO shaft. The clutch transmits a maximum torque between the input and output shafts in response to a maximum clutch pressure and transmits a variable torque between the input and output shafts in response to a given clutch engagement pressure that is less than the maximum clutch engagement pressure. Typically, a generally linear, gentle ramping up of current/pressure is employed to achieve smooth engagement.
The control system includes a first transducer disposed to generate an input signal representative of the rotational speed of the input shaft, a second transducer disposed to generate an output signal representative of the rotational speed of the output shaft, and a control circuit. The control circuit is coupled to the clutch control, the first transducer, and the second transducer.
While such a control system has been of great value and effectiveness, it and other control systems have continued to experience difficulties when attempts are made to drive PTOs under extreme conditions. With such systems, no differentiation was made with respect to the loads applied, be they very light or very heavy. With a light applied load, initial PTO shaft movement could occur at a relatively early time and the full shaft speed would be reached before a modulation is effectively executed. With a heavy load, however, initial PTO shaft movement would not occur until a later time, leaving very little time for modulation. Many systems sought a compromise under which load engagement worked reasonably well with intermediate loads, but less well with extreme situations, including situations where over-running clutches were associated with the PTO output shaft.
The strategy of employing a generally linear, ramping up of current/pressure to achieve smooth engagement, while generally relatively effective, nevertheless suffered from various shortcomings, especially under extreme load conditions, including use with associated over-running clutches.
The system of U.S. Pat. No. 6,267,189 addressed several of the remaining problems in greater detail, and explains in significant detail the problems encountered when over-running clutches are employed, including the possibility of placing undesirable stress on the PTO due to “locking delay” which occurs when the initially unengaged locking pins of an over-running clutch engage the locking notches thereof at a time subsequent to initial movement of the PTO output shaft. As was discussed therein, while the locking delay is of little concern at those times when PTO clutch is engaged, the locking delay may be problematic during engagement of PTO. In general, this is because a PTO clutch is modeled as ideally having two distinct operational states, (a) a first, disengaged state in which the plates of the clutch are not compressed and so the clutch does not transmit torque between the input shaft and the output shaft (and then to any connected load), and (b) a second, engaged state in which the plates of the clutch are compressed and the clutch transmits torque in an amount approximately directly related to the hydraulic fluid pressure applied to the clutch. However, in practice, a PTO clutch may still transmit a small but not negligible amount of torque from the input shaft to the output shaft even during the first, disengaged state, particularly if the hydraulic fluid pressure within the clutch is being increased to compress the plates and to cause the clutch to enter the engaged state. Even though this small amount of torque may be insufficient to rotate an PTO output shaft if equipment is directly loaded thereto, the torque may be sufficient to initially rotate an output shaft coupled as an input to an over-running clutch while locking pins 9 of the over-running clutch are disengaged from the locking notches thereof and until such time as the over-running clutch output locks to the over-running clutch input (i.e., while the transmitted torque may not be sufficient to rotate the a locked-up over-running clutch and its load, it may be sufficient to rotate the PTO output shaft during the locking delay). To summarize, the PTO clutch may transmit enough torque from the input shaft to the output shaft during the PTO engagement process, before the clutch is engaged, that the PTO will rotate from a position in which locking pins of the over-running clutch are disengaged from the locking notches to the position in which the locking pins are engaged with the locking notches.
Although U.S. Pat. No. 6,267,189 discussed in some considerable detail the problems posed by associated over-running clutches during PTO engagement operations, such patent's principal contribution to improved PTO clutch operation was directed less to the actual detection and controlled engagement of over-running clutches and more to an improved manner or strategy of overall PTO modulation. The system of such patent made use of a manner of adjusting the current increases to be applied to the clutch based upon comparisons made during the course of modulation of the actual acceleration and the desired acceleration, and basically presumed, based upon the fact that the mechanical parts between the PTO output shaft and the over-running clutch (i.e., the input shaft portion of the over-running clutch) could effectively be dragged to turn when the PTO clutch was only partially pressurized, that the actual acceleration would be very low when an over-running clutch was associated with the PTO output shaft. The system of such patent operated during modulation mode to increase the current at a slower rate when the acceleration was higher and at a faster rate when the acceleration was lower, except when the acceleration was found to be lower than some threshold, such as ⅙ of the desired acceleration, in which condition (presumptive indication of an over-running clutch) the increase in current was set to the lowest rate.
While such a strategy worked reasonably well in many cases, it had two significant drawbacks.
First, over-running clutches were quite commonly used with PTO driven implements, with different clutches exhibiting different kinds or degrees of “lock delay”. The strategy of U.S. Pat. No. 6,267,189 was generally more effective for over-running clutches that exhibited relatively small “lock delay”, but less so for over-running clutches that exhibited more pronounced “lock delays” and/or when the mechanical parts between the PTO output shaft and over-running clutch were light. In such cases, the detected acceleration would generally not only be higher than the threshold value, but higher than the normal acceleration encountered with non-over-running clutches and implements. Because the detected acceleration values did not fall within the “presumptive” category of over-running clutches, current increases would typically thus be applied at a higher rate than would be desirable for an over-running clutch, often resulting in abrupt engagements.
Second, when heavy loads and non-over-running clutches and implements were applied to the PTO output shaft, especially when operated at a low engine RPM, the actual PTO shaft acceleration could be so low as to be lower than the threshold, and such loading situations could then result in such loads being treated as “presumptive” over-running clutches when they were not. Under such condition, use of the strategy of U.S. Pat. No. 6,267,189 resulted in a very slow increase in current when, in actuality, it was preferred that current be more aggressively increased. Use of the strategy under such conditions thus often resulted in delayed and sluggish engagement and, in severe cases, failures to effect clutch lock-up within a desired or required time limit.
Thus, although the system of U.S. Pat. No. 6,267,189 significantly improved the overall manner in which PTO engagement is effected during the modulation period between initial movement of the output shaft and clutch lock-up and decreased the likelihood of encountering severe problems with extreme loads, and despite the advances realized through or as a result of the uses of such strategies, which have proven effective and beneficial in many instances, dealing with extreme load situations has remained troubling. In certain instances, the difficulties in effecting engagement could still result in either the application of very abrupt torque to a very light load, and the risk of possible damage thereto, or by a sudden and abrupt change in the load response due to locking delay when an over-running clutch is in use, which, in severe cases, could include risks associated with breakage of the shaft, unsafe operation, or engine stall.